In other words, it is a document to register products and/or services to be purchased and documented against what conditions. Once the document is booked, it can be sent to the external supplier by mail or email and it can be prepaid if required.
Normally the "procurement" role is in charge of managing this stage of the chain which usually comes after "Requisition" creation and management.
However that's not always true, Purchase orders can also be created from the scratch without the need of a previously created purchase requisition.
Starting from here, you will learn how to create and manage "Purchase Orders" in Openbravo.